1. Field of the Invention
This invention relates to loudspeaker transducers, and in particular, the configuration of a loudspeaker magnet assembly for a low profile loudspeaker transducer.
2. Related Art
Sound reproduction devices such as loudspeakers are utilized in a broad range of applications in many distinct fields of technology, including both the consumer and industrial fields. Generally, loudspeakers consist of one or more driver units in a box. These driver units are typically known as “loudspeaker drivers,” “drivers,” “loudspeaker transducer,” or “transducers.” Loudspeaker transducers utilize a combination of mechanical and electrical components to convert electrical signals (representative of the sound) into mechanical energy that produces sound waves in an ambient sound field corresponding to the electrical signals. The variations of electric energy are converted into corresponding variations of acoustic energy (i.e., sound waves) by rapidly vibrating a flexible diaphragm within the transducer.
Loudspeakers transducers are generally of two common construction types. The first construction type is a conventional dual-suspension driver construction where the diaphragm of the loudspeaker transducer is formed as a cone and is substantially greater in diameter than the voice coil. As an example, in FIGS. 1A and 1B, a typical known dual-suspension loudspeaker transducer 100 is shown. FIG. 1A shows a perspective view of the known loudspeaker transducer 100 and FIG. 1B shows a cross-section view of the known loudspeaker transducer 100. The loudspeaker transducer 100 shown is an example of an implementation of a moving coil electrodynamic piston driver commonly also known as a “dynamic loudspeaker.” The known loudspeaker transducer 100 may include a diaphragm 102, frame 104, surround 106, front plate 108, magnet 110, back plate 112, voice coil 114, former 116, center pole 118, vent 120, gap 122, spider 124, and optional dust cap 126.
In this example, the loudspeaker transducer 100 consists of the diaphragm 102 (also known as a “cone”) attached to the frame 104 (also known as a “basket”) via the surround 106. Attached to the rear end of the diaphragm 102 is a coil of wire (known as the voice coil 114) that is wound around a cylindrical extension of the diaphragm 102 that is known as the former 116. It is appreciated by those skilled in the art that in practice, the combination of both the voice coil 114 and former 116 may also be referred to as simply the “voice coil.” The former 116 is connected to the frame 104 via the spider 124. The combination of the surround 106 and spider 124 form a suspension system for the diaphragm 102. Both the spider 124 and the surround 106 generally act as a rim, made of flexible material that spans between the former 122 and the frame 104 and the diaphragm 102 and the frame 104, respectively. The suspension system acts to provide the stiffness of the diaphragm 102 and also provide air sealing for the transducer 100. The configuration of the voice coil 114, former 122, and diaphragm 102 in the frame 104 via the suspension system depends generally upon the design and size of the diaphragm 102 relative to the voice coil 114 and former 122. In an example of operation, the diaphragm 102 acts as a piston to pump air and create sound waves.
The loudspeaker transducer 100 also consists of the magnet 110, front plate 108, back plate 112, and center pole 118 (also known as a “pole piece”). The front plate 108, back plate 112, and center pole 118 are usually made of iron, steel, or a similar permeable material to form a magnetic circuit with the magnet 110, which is generally a permanent magnet. Typically, both the front plate 108 and back plate 112 are ring shaped. The magnet 110 is cylindrically ring shaped and the center pole 118 is a hollow cylinder that is located within the magnet 110 and extends between the front plate 108 and back plate 112. The center pole 118 has a lip at end that extends to the front plate 108 that is approximately perpendicular to center pole 118. The lip extends outward from the center pole 118 to the front plate 108 to form the gap 122. Generally, the front plate 108 and center pole 118 form the circular gap 122 of the magnetic circuit. The voice coil 114 and former 116 are then suspended within the gap 122 and spider 124 acts to center the former 116 and voice coil 114 within the gap 122 while also allowing former 116 and voice coil 114 to move freely back forth within the gap 122. The center pole 118 may include an optional cylindrical vent 120 that to prevent pressure from building behind the diaphragm 102 in the magnetic assembly and to provide for cooling of the voice coil 114. If the vent 120 is present, the optional dust cap 126 (also known as a “screen”) may also be present to prevent debris from entering through the vent 120.
In an example of operation, when an electrical signal from an amplifier passes through the voice coil 114, the voice coil 114 and former 122 turn into an electromagnet. Depending on which way the current is travelling in the voice coil 114, the north and south pole of the magnetic field, created by the voice coil 114, will be at one end of the voice coil 114 or the other. The magnet 110 has a north and south pole as well and its magnetic field will push the voice coil 114 (and the attached diaphragm 102) outward if the north and south poles of the two magnetic fields are lined up together (north-to-north and south-to-south) or pull the voice coil 114 inward if they are lined up oppositely (north-to-south and south-to-north).
The second type of driver construction is an edge-driven-diaphragm driver. In this construction, the diaphragm and the voice coil are of substantially equal diameter. The outer edge of the diaphragm is then attached to the diaphragm to form a diaphragm assembly. This assembly is then attached to the voice coil. The surround suspension assembly extends outward to connect the assembly to the frame. This edge-driven-diaphragm driver construction is often found in smaller speaker assemblies, such as tweeters, and sometimes in mid-range speakers. An example of edge-driven-diaphragm driver is described in U.S. Pat. No. 7,167,573, titled “FULL RANGE LOUDSPEAKER,” issued on Jan. 23, 2007 to inventor Clayton C. Williamson, which is hereby incorporated by reference in its entirety.
One common problem with smaller sized loudspeakers is as the size of the loudspeakers becomes smaller, achieving acceptable low frequency response becomes more difficult. This is because the loudspeaker is required to displace a larger volume of air to achieve the lower frequencies, and the suspension stiffness must be reduced to maintain a low resonance corresponding to the lighter mass of the smaller driver. The volume of air that a loudspeaker can displace is dependent upon the area of the diaphragm and the range of motion allowed by the suspension, i.e., amount of vibrational excursion, or volume displacement, of the loudspeaker. Additionally, higher suspension stiffness acts to reduce the motion of the diaphragm for a given input, so a minimum of stiffness is desired. Since smaller loudspeakers have a smaller diaphragm and stiffer suspension, the volume displacement, and thus the performance, is limited by the ability to manufacture loudspeakers with very low stiffness and high excursion capabilities.
To operate efficiently, the suspension system in smaller loudspeakers, such as those found in edge-driven diaphragm speakers, must allow a required maximum amplitude of vibration while constraining the vibrational movement essentially to a straight-line path to avoid the voice coil contacting the surrounding structure. Thus, the surround suspension member is required to constrain the diaphragm against any tilting, rocking or other extraneous vibration while allowing maximum possible amplitude of desired vibration. A general problem with the current construction of edge-driven speakers is the difficulty of precisely aligning the components during manufacturing, as the magnetic air gap is shielded by the diaphragm. This forces the removal of all alignment gauges prior to the placement of the diaphragm/coil assembly, and thus causes uncertainty in location of the voice coil relative to the motor. This is commonly known as a “blind” assembly.
An additional general problem with the current construction of loudspeakers is that spurious vibration of portions of the surround suspension members occur at high audio frequencies. These spurious vibrations may be transmitted to the diaphragm through the suspension, thereby degrading the high frequency performance of the speakers. Also, with the current loudspeaker construction, the maximum amplitude of vibration is limited in smaller sized loudspeakers, preventing low frequency responses from the smaller diameter speakers. Furthermore, the frame construction of even smaller sized loudspeakers prevents these loudspeakers from being thin enough for use in laptops and to electronic tablet devices.
A need therefore exists for a loudspeaker construction that minimizes the effect of the spurious vibration of the suspension system on the diaphragm, increases the amount of excursion of the voice coil/diaphragm assembly to provide low frequency response in smaller diameter loudspeaker systems, and has a low profile suitable for use in laptops, electronic tablet, and other low profile devices.